The present invention relates in general to the manufacture of electronic power module packages and, more particularly, to the bonding of materials having dissimilar coefficients of thermal expansion in the manufacturing process of the module package.
Electronic power modules incorporating power levels of 200 watts and more are used extensively in transportation, computer, HVAC, and other industry applications. These modules are popular due to their small feature size and comparative low cost. For additional cost reduction purposes, there is a demand to further increase the level of electronic integration within the module to provide increased functionality while maintaining the small feature size of the package. As a result of the increased functionality, the power dissipation and heat generation per unit area of the module increases.
In the past the module has been fabricated by attaching semiconductor die to one or more ceramic substrates and further bonding the resulting assembly to a copper base plate that serves as a combinational mechanical base and heat sink. The ceramic substrate is typically soldered to the base plate.
During manufacture of the module, the ceramic substrate and copper base plate assembly is heated to a temperature that assures the solder will bond adequately to both materials. When the assembly cools a stress develops along the solder bond because of the dissimilar thermal characteristics of the ceramic and copper materials causing the materials to expand and contract by different amounts. The stress exerted by the bonded materials may cause cracking or other failure of the solder bond and create hot spots in the module upon application of electrical power. Consequently, premature electrical failure of the module may occur.
Attempts have been made to solve the problem by plastically preforming the base plate so as to match the thermal expansion and contraction of the ceramic substrate. The copper base plate is bent typically at room temperature with an external force beyond its elastic range to create a permanent bow in the material. Unfortunately, it is difficult to precise and consistently bend the copper base plate beyond its elastic range so that when the external force is removed, the base plate retains the proper bow that will match the thermal expansion characteristics of the ceramic substrate.
The plastic pre-bow curvature is difficult to control because the shape of the curve depends on the plastic properties of the copper base plate and is inherently sensitive to variations in raw material properties. Moreover, each application typically requires its own, often expensive, stamping tool to achieve a specific pre-bow. Thus, variations in module base plate size, thickness, plasticity, and cooling techniques make it difficult to achieve the proper plastic bow in the base plate that matches the thermal expansion of the ceramic substrate.
Hence, a need exists for a module fabrication method that compensates for materials having dissimilar thermal expansion characteristics.